What is the difference between bpy.props.CollectionProperty and bpy.props.PointerProperty?
A collection is an array of a set of properties. Every element in the array has the same properties (with individual values).

A pointer property is used to organize properties. The sub-attribute the pointer property is stored at will hold a set of properties. It is not an array of properties however.

Scene.collection
  Scene.collection.add()
  Scene.collection.remove()
  Scene.collection ...()
  Scene.collection[0]
    Scene.collection[0].name # Foo
    Scene.collection[0].val # 123
  Scene.collection[1]
    Scene.collection[1].name # Bar
    Scene.collection[1].val # 234
  Scene.collection[...]
    Scene.collection[...].name # Baz
    Scene.collection[...].val # 345

Scene.pointer
  Scene.pointer.name # Foo
  Scene.pointer.val # 123